Prior to the present invention, virtually all absorbable sutures used in animal and human surgery were prepared from mammalian intestines, such sutures being commonly called catgut sutures. U.S. Pat. No. 3,297,033 describes an absorbable surgical suture made from polyglycolic acid. The disclosure therein is incorporated by reference. This patent in column 3, lines 20 to 53 disclose other components which may be present in the suture. As set forth therein, polyglycolic acid is also properly named as poly(hydroxyacetic acid) or poly-hydroxyacetic ester or polyglycolide and can be considered as essentially a product of polymerization of glycolic acid, that is, hydroxyacetic acid, which in simplified form is shown by the equation: ##STR1##
For use as a suture, preferably n is such that the molecular weight is in the range of 10,000 or more. Above 100,000 the polymer is difficult to extrude.
In these molecular weight ranges the polymer has a melt viscosity at 245.degree. C. of between about 400 and about 27,000 poises. Because the fiber is from a synthetic and controllable source, with a controlled molecular weight and is a homopolymer or has a controllable small percentage of comonomer, the absorbability, stiffness and other characteristics can be modified. In general, the higher the molecular weight, the slower the rate of absorption under a given set of conditions.
Among several methods by which polyhydroxyacetic ester can be prepared, one preferred route involves the polymerization of glycolide. ##STR2## the cyclic dimeric condensation product formed by dehydrating hydroxyacetic acid. During polymerization of glycolide, the ring is broken and straight-chain polymerization occurs. Probably at least a small portion of the polymerization involves the formation of anhydride or ether linkages from a condensation of glycolic acid in a head-to-head, or tail-to-tail direction. The current state of the art is not sufficiently advanced to show with certainty the ratio of anhydride or ether linkages to ester group but indicates these are no more than a few percent of the total. A small quantity of methoxyacetic acid, ##STR3## or methyl hydroxyacetic ester, ##STR4## or their homologs, such as higher alkoxyacetic acids, or alkyl hydroxyacetic esters may be present during the polymerization as an end group stabilizer controlling the molecular weight and viscosity. Small quantities of other materials may be present in the chain, as for example d,l-lactic acid, its optically active forms, homologs, and analogs.
Said U.S. Pat. No. 3,297,033 incorporates a reference to U.S. Pat Pat. 2,668,162 -- Lowe which quantifies a small amount of lactides as up to 15%, disclosing for example the preparation of a copolymer of 90/10 glycolide/lactide offers two advantages over the homopolymer of glycolide. One advantage is that the melting point of the copolymer is lower than the homopolymer, being in the neighborhood of 200.degree. C.; and the entire reaction can be conducted at approximately the melting point of the copolymer. Operation at the lower temperatures decreases the rate of degradation of the polymer which gives a polymer of lighter color. Another advantage is that the copolymer can be successfully quenched when being extruded into film because the copolymer is less crystalline. On the other hand, the homopolymer shows a greaer tendency to crystallize on extrusion and thereby tends to form opaque areas in the film.
Example 4 of said U.S. Pat. No. 2,668,162 shows reaction conditions.
Surgical elements of polyglycolic acid, including sutures, and other elements mentioned below can be better seen in most surgical fields if the element is colored so as to contrast with blood and tissue or bandages or other background materials.